Zirconium hydroxides and related compounds (e.g., titanium hydroxide, hafnium hydroxide) have been employed for isolation and/separation of selected ionic species under certain binding conditions, and depending on the particular ionic species, such compounds may have superior separation characteristics when compared to ionic exchange resins.
For example, U.S. Pat. No. 4,362,626 to Takeuchi et al. describes a preparation in which zirconium hydroxide is mixed with an epoxy resin, an unsaturated polyurethane, or polyethylene, and cured to form an adsorbent material. Takeuchi et al. use such materials to isolate phosphate anions from an aqueous solution at relatively highly acid pH (pH 3.0). Similarly, Echigo et al. describe in U.S. Pat. No. 4,576,969 an ion exchange resin in which a crosslinked phenolic resin encloses a water insoluble metal hydroxide, and wherein the inventors isolate cationic arsenic (As3+) from an aqueous medium at relatively acidic pH (pH 6.0).
In another example, as described in U.S. Pat. No. 5,578,218 to Matsuoka et al. the inventors utilize a polymeric resin that carries zirconium hydroxide to remove sulfate anions from a brine at relatively highly acidic pH (typically between pH 1.0-5.0, and preferably between 2.0 and 3.0). Alternatively, sulfate anions may be removed using zirconium hydroxide-coated activated carbon as described in U.S. Pat. No. 5,948,265 to Wakamatsu et al., and Shiga et al describe in U.S. Pat. No. 5,071,563 the use of a zirconium hydroxide slurry to bind sulfate wherein both, Wakamatsu and Shiga, employ relatively strong acidic conditions (e.g., pH 3.0-5.0) to bind the sulfate anion to their materials. Similar conditions and compositions are described to Joubert in U.S. Pat. No. 5,536,415.
In a still further example, as described in U.S. Pat. No. 6,238,544 to Oohara et al., iodine anions (e.g., iodate and periodate) are removed from an aqueous solution to be used for electrolysis by contacting the solution with a metal hydroxide under strongly acidic conditions (pH 3.0), wherein the metal hydroxide is (at least in some cases) disposed on a cation exchange resin. While most of the resin-metal hydroxide combinations may be particularly advantageous for column separation (e.g., desirable flow rates), the ion binding capacity of the metal hydroxide is often significantly reduced by the resin portion.
In order to circumvent at least some of the problems associated with reduced ion binding capacity of resin-bound metal hydroxides, Clarke et al. describe the use of metal hydroxides pastes and slurries for the removal and/or isolation of selected anionic species (arsenate, chromate, selenate, borate, fluoride, and perchlorate) under substantially neutral conditions (pH 6.0-7.5) using a layer of the metal hydroxide in a configurations with a high aspect ratio. In yet another example, as described in U.S. Pat. No. 4,855,059 to Frianeza-Kullberg, metallic cations (Ca2+ and Mg2+) are isolated from an alkaline aqueous solution using a metal hydroxide.
However, while most of the known metal hydroxide compositions and methods exhibit at least some advantage over other known materials to isolate cationic or anionic species from a medium, the pH of such media is generally limited to acidic (or at best neutral) pH.
Therefore, although there are numerous uses for metal hydroxides known in the art, all or almost all of them suffer from one or more disadvantages. Consequently, there is still a need to provide improved compositions and methods for metal hydroxides, and especially for adsorption of anions under alkaline conditions.